Climate control systems used in the passenger cabins of automotive vehicles can be classified into two types: simple and advanced. The two types use different signals to control the speed of a blower.
In a simple system, blower speed is controlled by an analog voltage which is controlled by the driver of the vehicle. That is, the driver controls blower speed directly. This analog voltage can be produced by a variable resistor.
In an advanced system, a digital control, rather than the driver, controls the blower speed directly. Further, the digital control does so by producing digital signals, rather than analog signals.
It is preferred that both the simple and advanced systems use the same type of blower. One reason is to reduce the number of types of blowers which the manufacturer must maintain in inventory. Clearly, this blower must be controllable by both the digital and the analog signals discussed above.
Interface circuits have been designed to convert both types of signal to a common signal which is intelligible to the blower. However, many of these interface circuits appear to be expensive, as well as subject to errors induced by variations in (1) power supply voltage and (2) ground offset voltages.